Be Professional In Professional Life

If you are expecting someone to spend much more than their regular spending for anything, then make sure that, you would make it completely voluntary. Don't force them to participate and pay.

If you are expecting the participants to pay for some activity that you have initiated, make sure that, you let them know in advance that they have to pay. If you don't let them know, there is nothing more unprofessional than that. If you have forgotten, that is completely your problem.

If you do like this, next time, don't expect anybody to listen to you.